介绍

Physical volume (PV): 物理卷,例如一个硬盘,或一个Software RAID设备; 硬盘的一个分区 (或者甚至硬盘本身或者回环文件)，在它上面可以建立卷组。It has a special header and is divided into physical extents. Think of physical volumes as big building blocks which can be used to build your hard drive.

Volume group (VG): 卷组,将一组物理卷收集为一个管理单元;Group of physical volumes that are used as storage volume (as one disk). They contain logical volumes. Think of volume groups as hard drives.

Logical volume(LV): 逻辑卷,等同于传统分区,可看作便准的块设备,以容纳文件系统;A "virtual/logical partition" that resides in a volume group and is composed of physical extents. Think of logical volumes as normal partitions.

Physical extent (PE): 物理块,划分物理卷的数据块;A small part of a disk (usually 4MB) that can be assigned to a logical Volume. Think of physical extents as parts of disks that can be allocated to any partition.

使用 LVM 你可以比正常的硬盘分区更容易的管理硬盘分区（逻辑卷）。例如，你可以：

使用卷组(VG)，使众多硬盘空间看起来像一个大硬盘。

使用逻辑卷（LV），可以创建跨越众多硬盘空间的分区。

可以根据需要，对分区（LV）和硬盘空间（VG）进行创建、删除、调整大小等操作。(it doesn't depend on position of the logical volumes within volume groups as with normal partitions)

Resize/create/delete partitions(LV) and disks(VG) online (filesystems on them still need to be resized, but some support online resizing)

Name your disks(VG) and partitions(LV) as you like

Create small partitions(LV) and resize them "dynamically" as they get more filled (growing must be still done by hand, but you can do it online with some filesystems)

在 LVM 上安装 Arch Linux

创建 LVM 分区

接下来，要创建LVM将使用的分区。文件类型使用'Linux LVM'，所以使用分区id 0x8e (文件系统类型：8E)。在需要使用LVM的每块硬盘上，各创建一个LVM分区。 Your logical volumes will reside inside these partitions so size them accordingly. If you will use only LVM and no other external partitions, use all of free space on each disk.

注意: You may need to load the device-mapper kernel module (modprobe dm-mod) for the above commands to succeed.

提示： 一开始可以创建小一点的逻辑卷lv，然后留一部分未使用空间在卷组vg里，以后可以根据需要再扩展各个逻辑卷。

建立文件系统与挂载逻辑卷

Your logical volumes should now be located in /dev/mapper/ and /dev/YourVolumeGroupName. If you can't find them use the next commands to bring up the module for creating device nodes and to make volume groups availabile:

# modprobe dm-mod
# vgscan
# vgchange -ay

Now you can create filesystems on logical volumes and mount them as normal partitions (if you are installing Arch linux, skip this step):

For early-boot activation of volume groups (namely for root filesystem containers), you'll need to make sure the lvm2 and udevmkinitcpio hooks are enabled.

/etc/mkinitcpio.conf:

HOOKS="base udev ... lvm2 filesystems"

Also make sure the dm_mod module is enabled.

/etc/mkinitcpio.conf:

MODULES="dm_mod ..."

You will need to rebuild the initramfs to commit any changes you made.

配置

扩大逻辑卷

To grow a logical volume you first need to grow the logical volume and then the filesystem to use the newly created free space. Let's say we have a logical volume of 15GB with ext3 on it and we want to grow it to 20G. We need to do the following steps:

If you want to fill all the free space on a volume group use the next command:

# lvextend -l +100%FREE VolGroup00/lvolhome

警告: 并非所有的文件系统都支持无损或在线扩大逻辑卷。

注意: If you do not resize your filesystem, you will still have a volume with the same size as before (volume will be bigger but partly unused).

缩小逻辑卷

Because your filesystem is probably as big as the logical volume it resides on, you need to shrink the filesystem first and then shrink the logical volume. Depending on your filesystem, you may need to unmount it first. Let us say we have a logical volume of 15GB with ext3 on it and we want to shrink it to 10G. We need to do the following steps:

Here we shrunk the filesystem more than needed so that when we shrunk the logical volume we did not accidentally cut off the end of the filesystem. After that we normally grow the filesystem to fill all free space left on logical volume. You may use lvresize instead of lvreduce.

Warning:

Do not reduce the filesystem size to less than the amount of space occupied by data or you risk data loss.

Not all filesystems support shrinking without loss of data and/or shrinking online.

Note: It is better to reduce the filesystem to a smaller size than the logical volume, so that after resizing the logical volume, we do not accidentally cut off some data from the end of the filesystem.

Remove logical volume

Warning: Before you remove a logical volume, make sure to move all data that you want to keep somewhere else, otherwise it will be lost!

First, find out the name of the logical volume you want to remove. You can get a list of all logical volumes installed on the system with:

# lvs

Next, look up the mountpoint for your chosen logical volume...:

$ df -h

... and unmount it:

# umount /your_mountpoint

Finally, remove the logical volume:

# lvremove /dev/yourVG/yourLV

Confirm by typing "y" and you are done.

Dont forget, to update /etc/fstab:

# sudo nano /etc/fstab

You can verify the removal of your logical volume by typing "lvs" as root again (see first step of this section).

添加分区到卷组中

To add a partition to your volume group you must first make its type 'Linux LVM' (for example with cfdisk). Then you need to create a physical volume on it and extend the volume group over it:

# pvcreate /dev/sdb1
# vgextend VolGroup00 /dev/sdb1

Now you have free space in your volume group that can be used by logical volumes in this group.

Tip: You can add partitions from any disks to volume groups.

从卷组中移除卷

All of the data on that partition needs to be moved to another partition. Fortunately, LVM makes this easy:

# pvmove /dev/sdb1

If you want to have the data on a specific physical volume, specify that as the second argument to pvmove:

# pvmove /dev/sdb1 /dev/sdf1

Then the physical volume needs to be removed from the volume group:

# vgreduce myVg /dev/sdb1

Or remove all empty physical volumes:

# vgreduce --all vg0

And lastly, if you want to use the partition for something else, and want to avoid LVM thinking that the partition is a physical volume:

# pvremove /dev/sdb1

快照功能

介绍

LVM可以给系统创建一个快照，由于使用了COW (copy-on-write) 策略，相比传统的备份更有效率。 The initial snapshot you take simply contains hard-links to the inodes of your actual data. So long as your data remains unchanged, the snapshot merely contains its inode pointers and not the data itself. Whenever you modify a file or directory that the snapshot points to, LVM automatically clones the data, the old copy referenced by the snapshot, and the new copy referenced by your active system. 这样的话，如果你只修改了不超过2G数据（包括原始的和快照的），你将可以只使用2G的空间，就能快照一个有35G的数据的系统。

配置

You create snapshot logical volumes just like normal ones.

# lvcreate --size 100M --snapshot --name snap01 /dev/mapper/vg0-pv

With that volume, you may modify less than 100M of data, before the snapshot volume fills up.

snapshots are primarily used to provide a frozen copy of a filesystem to make backups; a backup taking two hours provides a more consistent image of the filesystem than directly backing up the partition.

常见问题

LVM 命令不起作用

try preceeding commands with lvm like this:

# lvm pvdisplay

设定文件系统挂载点的页面不显示逻辑卷

If you are installing on a system where there is an existing volume group, you may find that even after doing "modprobe dm-mod" you don't see the list of logical volumes.

In this case, you may also need to do:

# vgchange -ay <volgroup>

in order to activate the volume group and make the logical volumes available.

Receiving Input/Output Errors after plugging in a removable device with LVM partitions

Symptoms:

~$ sudo vgscan
Reading all physical volumes. This may take a while...
/dev/backupdrive1/backup: read failed after 0 of 4096 at 319836585984: Input/output error
/dev/backupdrive1/backup: read failed after 0 of 4096 at 319836643328: Input/output error
/dev/backupdrive1/backup: read failed after 0 of 4096 at 0: Input/output error
/dev/backupdrive1/backup: read failed after 0 of 4096 at 4096: Input/output error
Found volume group "backupdrive1" using metadata type lvm2
Found volume group "networkdrive" using metadata type lvm2

Cause:

Removing an external LVM drive without deactivating the volume group(s) first. Before you disconnect, make sure to:

# vgchange -an <volume group name>

Fix: (assuming you already tried to activate the volume group with vgchange -ay <vg>, and are receiving the Input/output errors